ZLY 103 Animal Diversity

PHYLUM PLATYHELMINTHES The Acoelomates

Introduction•The first triploblastic animals with a third tissue layer, mesoderm; between the endoderm and endoderm.•Due to absence of a coelom or pseudocoel, they are termed acoelomate•They are flat in shape and possess a blind gut (mouth without anus).

•Size varies from a millimeter to several meters.•Some are free-living (Turbellaria) and parasitic.•Majority are bottom-dwellers in aquatic & moist lands.

Introduction• Eumetazoa - animals with tissue–Bilateria - have bilateral symmetry

and are triploblastic.–Protostomia - a group of animals

whose mouth develops from the blastopore, and the mesoderm forms from an area near the blastopore.–Acoelomata - They have a true

mesoderm which fills the original blastocoel between the outer epidermis and digestive tract.

Characteristics• They are triploblastic, acoelomate, non

metamerically segmented metazoans.• They are at organ system level of

organization.• They exhibit bilateral symmetry,• Body contains no internal cavity and

absence of blood system. • Protonephridial excretory organ (Flame

cells) instead of anus.• Nervous system made up of longitudinal

fibers instead of a nerve net.• Dorsoventrally flattened.

Characteristics• They carry out hermaphroditic reproduction.• Free-living forms feed on small animals and

other life forms.• Parasitic forms feed on nutrients derived from

the host tissues.• Some species especially the free-living occur

in major biomes; marine, freshwater & moist lands.

• Central nervous system consists of a brain and two or more ganglionated nerve cord.

• They include tape worms, flatworms, flukes and planarians etc.

• Highly successful, with about 25,000 species.

Characteristics• Shows a degree of cephalization• Epidermis soft and ciliated or covered by cuticle and

with external suckers or hooks or both• Simple sense organs with eyespot in some species.• Parasitic species normally move between different

habitats during their developmental stages.• A few species are commensals deriving harmonious

existence with another larger organism.• Absence of fossil records due to their soft bodies but

they are thought to have evolved around 550my ago.

Triploblastic Condition• The embryo divides into three instead of

two as it is in cnidarians (diploblastic).• With the presence of a (parenchyma)

mesoderm, the platyhelminthes:Have an increase in size,A separation of the alimentary canal from the

body wall, A variety of organs are now formed,There is a movement towards the organ-

system level of organization,Presence of developing muscles, Cilia and

flagella are becoming too irrelevant for the larger sizes evolving.

The Acoelomate Condition• The large size of platyhelminthes with the

emergence of the mesoderm (next slide) created the problems of efficient transport system, hence the acoelomate condition.

• The animals in this group do not have a choice but to be dorsoventrally flattened for it to present a large surface area for diffusion to satisfy the metabolic requirements (flatworms).

Internal section through the gut

Plate 1: Transverse section through a generalized Acoelomate

Epidermis/ECTODERM

gut cavity

no body cavity; region between gut and body wall packed with organs within MESODERM tissue

Acoelomate organism(flatworm)

Lining of gut =ENDODERM

Body plan

Body design in Platyhelminthes

Taxonomy/Classification

• Class Turbellaria • Dugesia

• Class Monogenea –Gyrodactylus

• Class Trematoda –Clonorchis, Fasciola, Schistosoma

• Class Cestoda –Taenia, Dipylidium, Echinococcus

Distinct Features of the Classes in Flatworms

• Proglotids and scolex in Cestoda• Rabdites in Turbellaria• Combination of characteristics-

protonephridia for excretion and acoelomates in all except Cestodes

• Larva of each class are unique–Turbellaria - Planaria–Cestodes - Oncosphere/Cysticerci

(bladder worm)

–Trematodes - Metacercaria/Cerceria

Distinct Features of the Classes in Flatworms

• IntegumentsInteguments- Rhabdites and one cell layer epidermis in Turbellaria and usually ciliated; syncytial tegument in other classes.

• SkeletalSkeletal - hydrostatic• MuscleMuscle - longitudinal, transverse, and

circular muscles are present.• DigestiveDigestive - incomplete with

intracellular and extracellular digestion; no system in Cestoda.

Distinct Features of the Classes in Flatworms

• Excretory Excretory - flame cells, or excretory tubes in Cestodes.

• RespiratoryRespiratory - no system, through diffusion

• CirculatoryCirculatory - none, diffusion.• NervousNervous - anterior ganglia, ventral

ladder-like system (two lateral cords with transverse cords).

Distinct Features of the Classes in Flatworms

• Endocrine Endocrine - hormones produced by nervous system

• ReproductiveReproductive - monoecious in most–Well developed reproductive organs,

mostly internal fertilization.–Two of the parasitic classes have

complex life cycles• Trematoda - miracidium, sporocyst, redia, cercaria, metacercaria (usually snail is secondary host.• Cestoda - oncosphere, cysticercus

Distinguishing Features among the Classes

Turbellaria Trematoda Monogenea CestodaFree living; aquatic

Endoparasitic  Ectoparasitic, larva is free-living

Endoparasiticor ectoparasitic 

Delicate, soft body

Leaf-like shape

 leaf-like/cylindrical in shape

Elongated, proglottides (able to detarch) 

Suckers rarely present

Ventral sucker plus sucker on ‘head’ for attachment to host

 posterior attachment with hooks, suckers or clamps

Suckers and hooks on ‘head’ plus sucker on ‘scolex for attachment

Enteron present

Enteron present

 Enteron absent

No enteron

Cuticle absent(outer surface covered with cilia)

Thick cuticle with spines (not parasitic)

 Syncytial teguments, no cilia

Thick cuticle (protection); no cilia in adult

Hermaphroditic/asexual fusion

Indirect development

 Direct development

Indirect development

Sense organs in adult

Sense organs only in free-living larval stages

Sense organs only in free-living larval stages

Sense organs only in free-living larval stages

e.g. Planaria, Dugesia, Planocera, Microstomum

e.g. Fasciola (liver fluke) Schistosoma (blood fluke)

 e.g. Dactylogyrus, Polystoma, Gyrodactylus

e.g. Taenia (tapeworm)

Turbellaria Trematoda Monogenea Cestoda

Reproduction in Platyhelminthes

Asexual by budding or transverse fission

•RegenerationSexual–Hermaphroditic; complex

reproductive system– Internal fertilization–Direct development, but some

polyclads have free-swimming larvae

Reproductive system of Platyhelminthes

Class Turbellaria• Free-living flatworms, mainly carnivorous.

• Mostly aquatic, a few terrestrial.

• Benthic (bottom dwellers).

• Variable in shape, size and features.

• Best example of cephalization.

• Surprising level of internal complexity

Class Turbellaria

• They are characterized by a soft epidermis ciliated at on the ventral surface.

• The movement of the cilia propels the smaller forms.

• Larger species glide along by muscular waves, usually over mucous beds secreted by special cells.

Turbellarians• Generally divided into five groups based

primarily on differences in the form of the digestive cavity.

• The most primitive are the acoels, (no

digestive cavity). • The ventral mouth, and sometimes a

simple pharynx, lead to an inner mass of nutritive cells.

• Most species measure less than 1/8 in. (3 mm) in length.

Planaria: Tubellaria

Turbellarians

• The rhabdocoels have straight, unbranched digestive cavities.

• Studies show that the rhabdocoels gave rise to both the trematodes and cestodes,

• Several rhabdocoel species exhibit commensal relationships, presumably led to parasitism.

• The allocoels were formerly classified together with the rhabdocoels; the gut can be either saclike or branched.

Diversity in Planarians

• The triclads, also known as planarians, are relatively large flatworms named for their three-branched gut (NEXT SLIDE).

• Most species range from 1/8 in. (3 mm) to about 1 in. (2.5 cm) in length.

• Planarians have more sense organs and a more complex brain than the other turbellarians (Polyclads).

Diversity in intestinal pattern of two orders of Tubellarians

Tricladida Polycladida

Biology of Turbellaria

• The freshwater species Dugesia tigrina has primitive eyes and tactile lobes, or auricles, on the sides of the head.

• Muscular pharynx can be extruded for food capture. 

• Dugesia and many other planarians can regenerate entirely new individuals from small pieces cut from the body.

Biology of Turbellaria• The group of turbellarians known as

polyclads tend to be larger (1–2 in./2.5–5 cm) and more oval-shaped than the triclads.

• Their bodies are extremely flat and leaf-like, and the gut is subdivided into numerous branches.

• Many are brightly colored and some have ruffled edges. Some species have numerous eyes scattered over the front end of the body.

Class Trematoda

• Flukes - oral and ventral suckers, no hooks, parasites, body with a syncytial tegument without cilia. Larva stages unique.

• Clonorchis

• Fasciola

• Schistosoma

Class Trematoda

E.g. Flukes • Most important is subclass

Digenea• 11,000 species; second most

diverse group of parasites (Nematoda#1)

• Endoparasites of vertebrates• Some cause debilitating diseases

to livestock and humans

Types of Hosts Often have complex life cycles that

alternate between sexual and asexual stages.

Most require at least 2 different kinds of hosts to complete their life cycle:

1. Definitive host (primary host) Host in which the parasite matures and

reproduces (sexually) Host in which eggs are released

2. Intermediate host Hosts in which larval stages develop and

undergo asexual reproduction Results in increase in the number of the

individuals

Clonorchis

• Human liver fluke - has two intermediate or secondary host snail and fish.

Cerceria

Adult human liver fluke

8.8

Clonorchis Life Cycle

Fasciola

Adult sheep liver fluke

metacercaria on grass.

Fasciola Body Plan

Life cycle of Fasciola

Schistosoma

• Blood Flukes Cerceria usually infect by burrowing through skin.

Schistosomiasis Schistosomiasis (bilharzia) is an infection with

blood flukes and is a major infectious diseases.

More then 200 million people are infected worldwide with these flukes.

They acquire infection by swimming or walking in water domiciled by the intermediate snail host.

Schistosome eggs enter the water when infected people urinate or defecate in or near water.

Schistosomiasis Eggs hatch and the miracidium seeks out a

snail. Within the snail, it develops into a sporocyst

and asexual reproduction takes place. Cercaria are eventually released into the water.

Cercaria swims and avoids UV light which can damage it, but is very sensitive to the scent of humans.

Certain molecules from human skin makes cercaria jerks and releases chemicals that soften the skin and burrows in shedding its tail at the same time (Schistosomula).

8.9a and b

Schistosome Life Cycle

Class Monogenea They were previously classified as an order

of the Trematoda, but recent work suggests they are more closely related to cestodes (tapeworms).

They are small (usually < 2cm) typically external parasites of fish that clamp onto the gills using a hooked organ (often with suckers) called an opisthaptor.

Some also parasitize the urinary bladder or rectum of frogs and turtles; there is a species that parasitizes squid and one that attaches to the eyeball of hippopotomuses.

Class Monogenea Unlike the trematodes, Monogeneans have only a

single host (hence “Mono” in the name). Most feed on the host’s epidermis using their protrusible pharynx, but some are blood feeders.

Monogeneans are hermaphrodites (male organs develop first) and can move around a host in search of a mate (they will also self-fertilize).

The egg hatches into a ciliated larva (an oncomiracidium) which seeks out its host in the water.

8.11

Monogenea Body Plan Gyrodactylus olsoni

Monogenean Fluke on gill of longjaw mudsucker fish

Class Cestoda (Tapeworms)• Have distinct head

structure (Scolex) and reproductive units called proglottids (Strobila).

• Absence of a digestive system.

• Larva are oncospheres and cysticerci (bladder worms) Taenia Dipylidium Echinococcus

Class Cestoda (tapeworms) They are parasites of the vertebrate

digestive tract and about 4000 species are known.

Almost all tapeworms require at least two hosts with the definitive host being a vertebrate, although intermediate hosts can be invertebrates.

They are quite different in appearance from the other members of the Platyhelminthes.

Cestodes Proglottids (Body Plan) & Head

Defini

ng C

hara

cter

istics

Class Cestoda New proglottids form behind the scolex and

the strobila may become extremely long.

Tapeworms live in the intestines where it is immersed in digested food therefore lack a digestive system. Instead they simply absorb food across their tegument.

To facilitate the absorption of food, the tegument has huge numbers of tiny projections called microtriches.

The surface area of the tegument for absorption is greatly increased.

8.12

Cestode: Taenia solium

Class Cestoda Tapeworms are usually monoecious

(have both male and female reproductive organs).

A proglottid is fertilized by another proglottid in the same or a different strobila.

Shell-encased embryos form in the uterus and exit the proglottid via a uterine pore or the entire proglottid may detach and pass out of the host.

8.15

Cestode: Life Cycle

Dipylidium (dog tape)• Juveniles in flea and

louse• Note the two

gential (red arrows) pores and reproductive systems in each proglottid.

Proglottid

Echinococcus• Hydatid cyst (cysticercus)- many scolices and

enlarges to the size of a basketball. • Often called sand.• Only three proglottides

Constraints Associated with Parasitic Living

• Reproduce within the definitive host• Get fertilized eggs out of the host• Contact a new and appropriate host• Obtain entrance into the host• Locate the appropriate environment within

host• Maintain position within the host• Withstand an often anaerobic environment• Avoid digestion or attack by the hosts

immune system• Avoid killing the host, at least until

reproduction is completed

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